Transfer Rater: Ander Herrera to Barcelona, Casemiro to United

The latest transfer rumours from around the world.

Things change in the blink of an eye in football. The Football Whispers Index takes the latest transfer rumours and gives them a score out of five; the higher the score, the more realistic and reliable the whisper.

Here are today's top five emerging whispers. And keep an eye on Transfer Talk for all the latest gossip.

Yaya Toure to New York City FC

Manchester City midfielder Toure may leave the club in the summer to join New York City FC. The Ivorian has been tipped to replace Andrea Pirlo at the MLS club after the Italian announced his intention to retire at the end of the season. Toure, 34, signed a one-year contract extension in June and determinedly stated his desire to work his way into Pep Guardiola's starting lineup. However, he has only managed two substitute appearances in the Premier League this season, with his only start coming in the Carabao Cup, and a move to City's sister club in New York would see him link up with former teammate Patrick Vieira, who is the head coach there.

Josh Onomah to Aston Villa

Aston Villa manager Steve Bruce is determined to sign on-loan Tottenham star Onomah. The 20-year-old has impressed during his spell at Villa Park and put in another eye-catching performance during the 2-1 win over Fulham on Saturday. Bruce admitted Villa would be required to "break the bank" for the England U21 star, but is aware Spurs have plans for him after his loan at the Championship side expires. "Josh Onomah was terrific. We'll try and break the bank for him! But we know Spurs have got a very good player there," Bruce said.

Ander Herrera to Barcelona

Barcelona boss Ernesto Valverde is keen on a reunion with Manchester United midfielder Herrera. The pair worked together at Athletic Bilbao and Valverde has identified the 28-year-old as a replacement for Andre Gomes, who looks to be on his way out of the Camp Nou. Herrera played under Valverde for one season at the Basque club before United spent £29 million on him in the summer of 2014. Although there is no indication of Herrera wanting to leave Old Trafford, he has not always been a guaranteed starter under Jose Mourinho this season.

Casemiro to Manchester United

Mourinho, meanwhile, has reportedly set his sights on Real Madrid midfielder Casemiro. The Brazilian has established himself as a key player under Zinedine Zidane at the Bernabeu and United have taken notice. However, they are likely to face stiff competition from Paris Saint-Germain for his services. Casemiro played against United in the UEFA Super Cup back in August, scoring in a 2-1 win for Madrid.

Alejandro Grimaldo to Manchester United

Another Manchester United target, according to reports in Portugal, is Benfica's Grimaldo. The left-back reportedly caught Mourinho's eye during United's 1-0 win Champions League win over Benfica last week. Left-back has been something of a problem position for Mourinho of late, with Ashley Young, Matteo Darmian and Daley Blind all having featured there. Marcos Rojo remains injured while Mourinho still has doubts over Luke Shaw, and would like to bolster his options by making a move for Grimaldo.